Retired Adm. Harry Harris, the nominee to be the U.S. ambassador to South Korea, said Thursday that North Korea still poses a nuclear threat to the United States despite President Donald Trump's Tweets to the contrary.

"The whole landscape has shifted" following the Singapore summit between Trump and North Korean leader Kim Jong Un, Harris said at his confirmation hearing before the Senate Foreign Relations Committee.

"I believe we should give exercises, major exercises, a pause to see if Kim Jong Un is in fact serious about his part of the negotiations, he added, citing Vice President Mike Pence in stating that joint training with the South Koreans could continue on a smaller scale.

In a Tweet on Wednesday, Trump said, "Everybody can now feel much safer than the day I took office. There is no longer a Nuclear Threat from North Korea."

However, Harris warned that North Korea, with an estimated 30 to 60 nuclear weapons and intercontinental ballistic missiles (ICBMs) capable of hitting the U.S. mainland, would remain a threat to the U.S. and its allies until the dismantling of its weapons programs can be verified.

"I think we must continue to worry about the nuclear threat," he said. "Until that is dismantled, I don't think we can rest comfortably."

The suspension of joint military exercises by U.S. and South Korean forces -- Trump called them "war games" -- was not included in the joint declaration signed by Trump and Kim at the Singapore summit.

Trump made the announcement of the freeze at a news conference following the summit and initially appeared to catch U.S. Forces Korea, the Pentagon and the South Koreans by surprise, although Defense Secretary Jim Mattis said through a spokeswoman that he had been told beforehand.

Secretary of State Mike Pompeo met privately for an hour in Seoul on Wednesday with Army Gen. Vincent Brooks, commander of U.S. Forces Korea, the Combined Command of the U.S. and South Korea, and the United Nations Command, and said later that the cessation of exercises would remain in effect as long as the North continued negotiating in good faith.

In Seoul on Thursday, South Korean President Moon Jae-in said in a statement issued by the presidential residence (the "Blue House") that his country would go along with the freeze on exercises "if North Korea implements denuclearization measures and sincere dialogue continues between South Korea and the North, the North and the United States, to ease their hostile relations."

Moon said South Korea "needs to flexibly change its military pressure against the North in the spirit of building mutual trust as agreed in the Panmunjom Declaration," a reference to his own earlier meeting with Kim at the truce village in the Demilitarized Zone separating the two Koreas.

He urged caution in predicting the outcome of continuing negotiations with the North on a range of issues.

In the lead-up to the Singapore summit, Moon said he had received assurances from Kim that the continuation of military exercises would not be a roadblock to negotiations.

In his questioning of Harris at the confirmation hearing, Sen. Bob Menendez, D-N.J., the ranking member of the Senate Foreign Relations Committee, said Trump's announcement of a freeze on military exercises had "blindsided" the U.S. military and South Korea.

Menendez told Harris, who was initially nominated to be ambassador to Australia, that he would be going to Seoul "at a time when our allies and adversaries are questioning our commitment to Asia."

"I think the president blindsided everyone including South Korea when he carelessly conceded to Kim Jong Un this week something North Korea has long wanted -- the cessation of South Korean joint military exercises [with the U.S.] in exchange for, well, apparently nothing," Menendez said.

Harris, who sported a new mustache and wore a Hawaiian lei as a former PACOM commander based in Hawaii, said, "In my previous capacity, I spoke very strongly about the need to continue military exercises, most notably in 2017.

"We were in a different place in 2017. North Korea was launching missiles," he said, and "if war wasn't imminent, it was likely. Today, following the [Singapore] summit, we are in a dramatically different place" and the freeze on exercises could encourage the North Koreans to disarm.

As ambassador, Harris said he would be wary of China's using the thaw with North Korea to press for the early lifting of sanctions against North Korea and the removal of the Army's THAAD (Terminal High Altitude Area Defense) anti-missile system from South Korea.

In Beijing on Thursday, Secretary of State Mike Pompeo said China had agreed not to press for quick sanctions relief.

Harris also said the U.S. and South Korea had to be focused in the long term on the removal of the threat posed by North Korea's massive array of conventional rocket and artillery emplacements along the DMZ.

"I think all of those things are on the table, but we start there" with North Korea's nuclear weapons, he said. "I think the administration has that realism at heart."

"I don't know how China will react" in the coming talks with North Korea, he said. "I do know based on my previous job that China is very unhappy with the placement of the THAAD system, but that was an alliance decision" between the U.S. and South Korea.

South Korea initially balked on the THAAD placement but ultimately allowed it last year despite the protests of local residents.

Harris said it is still in the U.S.'s national security interests to keep THAAD in South Korea to defend against North Korean tactical missile threats. THAAD was put in place "because of the threat from North Korea -- not for China, Russia or anything else."

THAAD was designed to counter short-range missiles and not the long-range threat posed by ICBMs, he said. If North Korea gave up its missiles, and the dismantling could be verified, "there would be no need for it."

Harris, who had held the title of "Old Goat" as the longest serving active-duty graduate of the Naval Academy, acknowledged that he has a steep learning curve ahead as a beginner diplomat.

"I fully appreciate that I will have to come up to speed quickly," he said. But after 40 years in the Navy, "I do understand diplomacy as an instrument of national power."